This invention relates to the communication over a telephone switching network of data between remote data terminals and a centralized data service such as a data processing center, or a data transmission network access point. In the remainder of this description, the term "processing center" will be used, which should be understood as "a transmission terminal point where many modems are typically used presently".
Typically the remote terminals produce serial digital data in a standardized format (e.g. RS232 C) which are converted by modems to analog signals for transmission to the local telephone switching office. In the telephone switching office analog signals from various different remote terminals may be time multiplexed together with telephone voice signals in a pulse code modulation (PCM) format and sent over a trunk to the telephone switching office near the data processing center. Typically the trunk accommodates 24 channels in North America, 30 channels in Europe. On these trunks, the signals are encoded in 8 bit words, via standard codes: mu-law in North America, A-law in Europe. At that remote switching office, the PCM signals are changed back to analog signals for transmission along separate analog loops to respective modems which convert the analog signals back into the initial serial digital data signals which were output from the remote terminals. The data processing center can read the signals in that form. The transmission of data back to the remote terminals is effected in a similar but, of course, reverse manner.
It should be apparent that in the conventional system described above the number of remote terminals which can access the data processing center at any particular time is limited by the number of modems connected between the data processing center and its associated telephone switching office and usually a large "pool" of these modems has to be provided.
It is an object of the present invention to replace this pool of modems with a single apparatus, thereby effecting considerable cost savings.